This invention pertains to the preparation of semiconductor materials and in particular to obtaining a lattice constant grading layer from a GaAs substrate to GaAs.sub.1-x Sb.sub.x.
The efficiency of a combination of photovoltaic cells is known to be a function of the choice of bandgaps for the respective cells. For a dual, stacked configuration an optimum choice of 1.1 ev and 1.65 ev has been shown for a standard solar spectrum. Implementation of such a multigap photovoltaic device is particularly attractive in the quaternary alloy system, AlGaAsSb for which a range of direct bandgap between 0.7 and 1.9 ev is readily achievable.
The fabrication of a multi-junction stacked device on a GaAs substrate introduces problems of lattice mismatch. The resulting defect density introduces losses which degrade the achievable efficiency for such devices. It is well known in the prior art to match unequal lattice constants of differing materials by growing an intermediate transition layer characterized by a gradually varying lattice constant. As employed herein, the term "lattice matched" is meant to convey that the lattice constants of the respective adjacent layers are matched to within 0.5%. Al.sub.y Ga.sub.1-y As.sub.1-x Sb.sub.x will be understood by the symbols AlGaAsSb for the purposes of this work.
Prior art lattice matching of GaAsSb on GaAs has been accomplished through step grading by growth of successive layers, each layer higher in Sb concentration than the underlying layer. Clearly this approach requires a considerable manufacturing effort.
Another prior art approach employs continuous grading by addition of a phosphorous (P) constituent. The gradual depletion of P leads to the desired lattice constant. Considerable accuracy in P concentration (of the order of 1 part in 10.sup.5) is required for initial lattice matched growth because of the small amounts of P involved.
An even greater disadvantage with this approach is inherent in the difficulty in maintaining reproducibility in manufacture owing to the effect on the GaAsSbP soluion from the high vapor pressure of P.
Still another prior art technique utilizes the growth of lattice matched layers of GaAsSb through substitution of Al for Ga while maintaining constant the concentration ratio As/Sb. This constant ratio maintenance in addition to the controlled variation of the Al/Ga ratio requires much process control equipment and supervision.
It is known, in the liquid phase epitaxy of a number of III-V binaries, that intermittent cooling reduces the dislocation density for the growth.